The term ‘deep sky’ has been used to cover a wide variety of dissimilar objects. Generally it means objects which are neither Solar system members nor stars. Deep sky can be broken out into two broad classes; non-stellar objects which are members of our own galaxy (nebulae) and external galaxies. The study of these faint, fuzzy patches of light actually has it roots in the search for comets. While Herschel and others noted the presence of whispy images, it was Charles Messier who catalogued them. In actuality, he was searching for comets and occasionally came upon small glowing clouds which looked like comets. In order to eliminate the false comets, which do not move with respect to the stars, he made a list of non-cometary extended glowing patches. In his day it was unclear that some of the objects were internal to our galaxy while others were external galaxies in their own right. Indeed, at the time, the true structure of our galaxy was only beginning to dawn on Herschel. Thus, the Messier Catalogue has external galaxies and internal galactic objects mixed together.

Messier's list of non-comets is shown in Appendix 10. There is some question about several of the objects. At least one, M1 02, is an accidental repetition of M1 01. M40 is just a double star. The last five or so objects were actually reported by Messier's colleague, Pierre Méchain.